Passivation effects of atomic-layer-deposited aluminum oxide

ICTEAM, Université catholique de Louvain, Place du Levant 3, 1348 Louvain-la-Neuve, Belgium

^a e-mail: Raja.Kotipalli@uclouvain.be

Received: 16 September 2012
Accepted: 24 June 2013
Published online: 27 September 2013

Abstract

Atomic-layer-deposited (ALD) aluminum oxide (Al₂O₃) has recently demonstrated an excellent surface passivation for both n- and p-type c-Si solar cells thanks to the presence of high negative fixed charges (Q_f ~ 10¹²−10¹³ cm^-2) in combination with a low density of interface states (D_it). This paper investigates the passivation quality of thin (15 nm) Al₂O₃ films deposited by two different techniques: plasma-enhanced atomic layer deposition (PE-ALD) and Thermal atomic layer deposition (T-ALD). Other dielectric materials taken into account for comparison include: thermally-grown silicon dioxide (SiO₂) (20 nm), SiO₂ (20 nm) deposited by plasma-enhanced chemical vapour deposition (PECVD) and hydrogenated amorphous silicon nitride (a-SiNx:H) (20 nm) also deposited by PECVD. With the above-mentioned dielectric layers, Metal Insulator Semiconductor (MIS) capacitors were fabricated for Q_f and D_it extraction through Capacitance-Voltage-Conductance (C-V-G) measurements. In addition, lifetime measurements were carried out to evaluate the effective surface recombination velocity (SRV). The influence of extracted C-V-G parameters (Q_f,D_it) on the injection dependent lifetime measurements τ(Δn), and the dominant passivation mechanism involved have been discussed. Furthermore we have also studied the influence of the SiO₂ interfacial layer thickness between the Al₂O₃ and silicon surface on the field-effect passivation mechanism. It is shown that the field effect passivation in accumulation mode is more predominant when compared to surface defect passivation.